A retrodirective reflecting device is one which is capable of receiving an electromagnetic signal and retransmitting it to the source from whence it came. Retrodirectivity, that is, the capability of a device to have an outgoing wave travel in a direction exactly opposite that of the incoming wave, is the basis for a covert beam projector described in Statutory Invention Registration (SIR) H299, July 7, 1987. The covert beam projector described in SIR H299 utilizes the principle of phase conjugation in the optical region of the spectrum to achieve retrodirectivity. However, phase conjugation can also be achieved in the microwave and millimeter wave regions with the use of Van Atta array. Van Atta array is the subject of U.S. Pat. No. 2,908,002 and one-dimensional version of the array is shown in FIG. 1. The essential characteristic of a Van Atta array is that the transmission lines, 2, 4, 6, connecting each pair of antenna elements 8, 10, 12 respectively, cause the same phase delay in the electromagnetic wave in the pairs of antenna elements. Each antenna element in a pair is equidistant from the geometric center of the array. Under the equal delay principle, if the incoming wave is tilted so that it is incident first on the elements to the right of the center, the wave, through the equal delay transmission lines, will be retransmitted through the elements to the left of the center. Thus, the advanced signals on reception are retransmitted as delays, and vice versa. Hence, the sum of the retransmitted signals add coherently in the direction of the original signal source, expressed ##EQU1## where
E.sub.rt is the resultant retransmitted field,
X.sub.i.sup.(r) is the displacement of the i.sup.th receiving element from the array center,
X.sub.i.sup.(f) is the displacement of the transmitting element from the array center,
.theta..sub.r =the received angle of arrival from broadside,
.theta..sub.t =the retransmitted angle which is an independent variable,
.phi..sub.L =the common interconnecting line phase delay
.lambda.=wavelength of the beam
.omega.=frequency of the beam
t=time
Bidirectional amplifiers 14, 16, 18 may be included as shown in FIG. 1 in the transmission lines, but the phase delay through each amplifier must be exactly the same.
Several such line arrays can be combined to provide a circularly symmetrical array as shown in FIG. 2. In this array, retrodirective transmission is preserved regardless of the angle of incidence of the incoming wave on the array. For the sake of simplicity, the connecting transmission lines and the amplifiers are not shown.